This invention relates to a method for preventing oxygen from deactivating sulfactive reducing catalysts, and particularly catalysts comprising cobalt and molybdenum components on a refractory oxide support.
Catalysts comprising cobalt and molybdenum sulfides as the active catalytic components on essentially inert supports, such as alumina or other porous refractory oxides, are well known to be active for promoting reducing reactions, for example, the reduction of sulfur dioxide to hydrogen sulfide by reaction with hydrogen. As taught in U.S. Pat. Nos. 3,752,877 and 4,243,647, both of which are herein incorporated by reference in their entireties, such cobalt-molybdenum catalysts are also highly useful, under net reducing conditions, for not only reducing sulfur dioxide to hydrogen sulfide by reaction with hydrogen, but also mercaptans and sulfur vapor. In addition, the same catalyst is also useful for hydrolyzing carbonyl sulfide and carbon disulfide to hydrogen sulfide at elevated temperatures, usually above about 650.degree. F. Thus, for a Claus tail gas stream or the like, which contains an assortment of sulfur components such as SO.sub.2, CH.sub.3 SH, COS, CS.sub.2, and S.sub.x (vapor), cobalt-molybdenum and other Group VIII-Group VIB catalysts are highly useful for simultaneously converting in a single catalytic reactor vessel all of said components to H.sub.2 S. The H.sub.2 S is then usually treated separately for conversion to elemental sulfur as a product, for example, by the Stretford process mentioned in U.S. Pat. No. 3,752,877 or the partial oxidation process using vanadia catalysts taught in U.S. Pat. No. 4,243,647 or by the methods described in U.S. Pat. Nos. 4,283,379 and 4,325,936.
One known difficulty with using sulfided cobalt-molybdenum catalysts and the like for the simultaneous hydrogenation/hydrolysis reactions described above is that, if oxygen is present in the feed gas entering the catalytic chamber, the catalyst will gradually and irreversibly deactivate for the intended conversions to hydrogen sulfide and particularly for the reduction of SO.sub.2 to H.sub.2 S. See, for example, U.S. Pat. No. 4,060,589 at column 5, lines 6 to 13, and U.S. Pat. No. 4,243,647 at column 5, lines 65 to 68.
Thus, in the process described in the aforementioned U.S. Pat. No. 3,752,877, the reducing gas generator yielding the necessary reducing agents for the hydrogenation reactions by the partial combustion of a fuel gas is normally operated under sub-stoichiometric conditions. That is, the amount of oxygen (or air) fed to the generator is less than that required for full combustion of the fuel to CO.sub.2 and water vapor, with the intended result being that all the oxygen is consumed. However, in actual practice, even under sub-stoichiometric conditions, incomplete consumption of oxygen can occur, perhaps due to poor burner design or inefficient operation of the burner in the reducing gas generator. Additionally, even when these problems are overcome, upsets in burner operation can also result in oxygen being present from time to time with the desired reducing agents.
Therefore, it is a primary object of this invention to provide a method for preventing oxygen from deactivating or otherwise "poisoning" a hydrolysis and/or hydrogenation catalyst susceptible to oxygen deactivation.
It is an especial object of the invention to provide such a method for protecting sulfided cobalt-molybdenum catalysts employed to simultaneously convert assorted sulfur components to hydrogen sulfide.
Another object of the invention is to provide a process for essentially completely hydrolyzing carbonyl sulfide to hydrogen sulfide at temperatures less than about 600.degree. F.
And yet another object of the invention is to provide a fuel-saving process for hydrogenating any sulfur component selected from the group consisting of sulfur dioxide, gaseous mercaptans and sulfur vapor and hydrolyzing carbonyl sulfide at temperatures less than 600.degree. F., with the fuel savings increasing when oxygen is present in the feed.
These and other objects of the invention will become more apparent in view of the following specification and claims.